Due to its characteristics of being easily applicable to various products and electrical characteristics such as a high energy density, a secondary battery is not only commonly applied to a handheld device, but universally applied to an electric vehicle (EV) or a hybrid vehicle (HV) that is propelled by an electric motor. This secondary battery is gaining attention for its primary advantage of remarkably reducing the use of fossil fuels and not generating by-products from the use of energy, making it a new eco-friendly and energy-efficient source of energy.
Recently, with the increasing interest in smart grids, an energy storage system that stores excess energy is required. The smart grid is disclosed in Korean Patent Publication Nos. 10-2011-0134803 and 10-2012-0016767. The energy storage system performs charging and discharging repeatedly based on an energy demand of an electrical grid. In particular, because the energy demand of the electrical grid is quite variable over time, it is difficult to estimate which point in time the energy storage system will start to charge or discharge to store excess energy or provide the electrical grid with energy in an amount of energy consumption the electrical grid requires. Thus, the energy storage system needs to maintain a proper charge amount so that it may be ready to charge or discharge at any time.
More specifically, a description of operation of an energy storage system in an actual environment will be provided through illustration.
FIG. 1 is a graph illustrating changes in charge amount and discharge amount of an energy storage system over time.
FIG. 1 shows an example of an amount of charging power supplied to an energy storage system and an amount of discharging power outputted from the energy storage system at a certain time zone. In FIG. 1, a longitudinal axis indicates an amount of power, and an upper portion above a value of ‘0’ in the middle represents an amount of charging power supplied to the energy storage system and a lower portion represents an amount of discharging power outputted from the energy storage system. Accordingly, a charging period of the energy storage system is a period of time during which an amount of power consumed is less than an amount of power produced from a generator, and a discharging period of the energy storage system performs discharging is a period of time during which an amount of power consumed is more than an amount of power produced from the generator.
As shown in the illustration of FIG. 1, the energy storage system repeats charging and discharging continuously. The reason is, as described in the foregoing, that an amount of power consumption an electrical grid requires and an amount of excess power in the electrical grid unexpectedly varies over time. Assume that an energy storage system operates without separate charge/discharge control under the varying condition of an electrical grid as shown in the illustration of FIG. 1. However, for better understanding, an overall operating state of an energy storage system will be described through a change in amount of power charged in any one secondary battery included in the energy storage system.
FIG. 2 is a graph illustrating an amount of power charged in a secondary battery included in an energy storage system over time.
Referring to FIG. 2, a longitudinal axis indicates the voltage of the secondary battery. The secondary battery is an exemplary secondary battery having a voltage value of 3.7V when fully discharged and a voltage value of 4.2V when fully charged. Thus, a state of charge (SOC) corresponds to 100% at 4.2V and 0% at 3.7V. Also, for convenience of understanding, a proper charge amount of the secondary battery is assumed to correspond to 50% SOC at 4.0V.
From FIG. 2, it is found that an amount of power charged in the secondary battery gradually reduces over time. Accordingly, it can be analyzed that the exemplary situation shown in FIG. 1 corresponds to a situation in which an amount of power consumed is larger than an amount of excess power in an electrical grid to which the energy storage system is connected.
Nevertheless, as described in the foregoing, for the purpose of operation, the energy storage system needs to maintain a proper charge amount so that it may be ready to charge or discharge at any time. From this point of view, the secondary battery (or the energy storage system) does not keep a proper charge amount. Particularly, going into a point in time t′ in FIG. 3, it can be seen that the secondary battery reaches the voltage of 3.7V corresponding to 0% SOC. At this point in time, if an amount of power consumed in the electrical grid increases and the energy storage system is requested to supply energy, the energy storage system cannot respond to the energy supply request.
Meanwhile, to satisfy an amount of power consumption the electrical grid requires during a peak load (a period in which power consumption is highest), the energy storage system connected to the electrical grid requires a large scale enough for charging of a large amount of excess power to satisfy an amount of power consumption in response to the peak load. However, the peak load of the electrical grid has seasonal and temporal characteristics. Accordingly, with an aim to provide sufficient power supply for satisfying an amount of power consumption required during a peak load in a particular season or at a particular time (a relatively short time), increasing a scale of the energy storage system may be a considerable cost burden.
Therefore, to solve the problem, there is a need for an apparatus and method for maintaining a charge amount of the secondary battery included in the energy storage system.